SAR JUNE 2014

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ESTIMATORS & DESIGNERS THE RATIONAL DESIGN According to SANS1900 (Manufacture of monoplaner prefabricated timber roof trusses [nail-plated]), the design used to manufacture each roof truss in a batch or order shall be a rational design. The definition of a rational design means that a design is done by a process of calculations and reasoning, and will include, or use, as a basis, a code of practice or accepted published technical documentation. In the case of nail-plated roof trusses this will be attained by applying design software supplied by an accredited (software) system in South Africa. The design shall include the grade of timber, dimensions of timber, the geometric configuration of all the members and the dimensions and positioning of the nail plates, connectors and cleats to be used. It therefore stands to reason that estimators and designers of monoplaner roof trusses in South Africa not only need to know how to use the software provided by their system, but must also have an intimate knowledge, and be able to demonstrate their understanding, of the following: SANS 10160 and SANS 10163 – General Loading Code, and the basis of structural design and actions for buildings and industrial structures and code of practice for The Structural Use of Timber in the design of timber structures SANS 10243 – The Manufacture and Erection of Timber Roof Trusses The Application of the National Building Regulations (SANS 10400) The ITC-SA Roof Erectors Handbook for the Installation and Bracing of Pre-fabricated Timber Roof Trusses (Volume 2) It is highly recommended that both the specifier and consumer insist that the estimator and designer are able to demonstrate and prove their knowledge and ability in this regard. COVER STORY The application of SANS1900 and the minimum requirements of the ITC-SA in the design, manufacture and inspection of erected roof trusses "Productivity and efficiency can be achieved only step by step with sustained hard work, relentless attention to details and insistence on the highest standards of quality and performance.” – J.R.D. Tata JUNE 2014 Incorrect truss loading.

Transcript of SAR JUNE 2014

ESTIMATORS & DESIGNERS

THE RATIONAL DESIGN

According to SANS1900 (Manufacture of monoplaner prefabricated timber roof trusses [nail-plated]), the design used to manufacture each roof truss in a batch or order shall be a rational design.

The definition of a rational design means that a design is done by a process of calculations and reasoning, and will include, or use, as a basis, a code of practice or accepted published technical documentation. In the case of nail-plated roof trusses this will be attained by applying design software supplied by an accredited (software) system in South Africa.

The design shall include the grade of timber, dimensions of timber, the geometric configuration of all the members and the dimensions and positioning of the nail plates, connectors and cleats to be used.

It therefore stands to reason that estimators and designers of monoplaner roof trusses in South Africa not only need to know how to use the software provided by their system, but must also have an intimate knowledge, and be able to demonstrate their understanding, of the following:

• SANS 10160 and SANS 10163 – General Loading Code, and the basis of structural design and actions for buildings and industrial structures and code of practice for The Structural Use of Timber in the design of timber structures

• SANS 10243 – The Manufacture and Erection of Timber Roof Trusses

• The Application of the National Building Regulations (SANS 10400)

• The ITC-SA Roof Erectors Handbook for the Installation and Bracing of Pre-fabricated Timber Roof Trusses (Volume 2)

It is highly recommended that both the specifier and consumer insist that the estimator and designer are able to demonstrate and prove their knowledge and ability in this regard.

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The application of SANS1900

and the minimum requirements of the ITC-SA in the design,

manufacture and inspection of erected roof trusses

"Productivity and efficiency can be achieved only step by step with sustained hard work, relentless attention to details and insistence on

the highest standards of quality and performance.” – J.R.D. Tata

JUNE 2014

Incorrect truss loading.

The submission of a quotation on an official company letterhead is not necessarily sufficient proof of this.

An estimator and/or designer should be appointed by his employer (the fabricator) on a basis of proven competence, and the acceptance of an appointment letter where he accepts his responsibilities according to the aforementioned standards and regulations.

Furthermore, as a minimum standard, the estimator should have passed the basic designer course, and the designer the intermediate designer course, as laid down by the Institute for Timber Construction (ITC-SA).

In terms of the recent South African Qualifications Authority (SAQA) registration of the ITC-SA as a professional body, a roof designer will be issued with a certificate of competence by the ITC-SA which together with his fabricator appointment, will ratify his ability to undertake design work in terms of the fabricator's manufacturing category.

THE LETTER OF APPOINTMENT OF AN ESTIMATOR IN RESPECT OF THE PREPARATION OF QUOTATIONS WILL ENSURE:

• That all quotations are in writing with both the fabricator and client details.

• To ensure a traceable job number to at least the year of quotation.

• That the inclusion of both roof area and the roof plan are mandatory on the quotation and that the roof plan is to include, as a minimum, the fabricator details and COC number, client details, job number, design codes of relevance and loads applied.

• That number(s) and/or titles of the architect’s drawings that are used in the preparation of the quotations.

• Must state loadings, maximum truss centres, maximum batten/purlin centres, span, pitch, overhangs and any additional information required to define any unusual truss profiles. (In the event of there being a range of spans and/or pitches, the minimum and maximum spans and pitches shall be stated).

• State quantities of all bracing and runners required to provide stability to the trusses. Bracing shall be in accordance with either the ITC-SA requirements or any special requirements specified by the design engineers, or as laid down in SANS 10163 & SANS 10243.

• Details and quantities of all cleats, hurricane clips and hangers necessary to ensure compliance with the design requirements must be provided. Where specially fabricated cleats are required and their design or cost is not known, these may be excluded, but this exclusion must be clearly stated on the quotation.

• Where sundry timbers are quoted, the total quantities, sizes and purpose must be clearly stated.

• The estimator must check that any member sizes and grades specified by the client or his architect for any trusses, beams or joists are adequate. If any sizes are found to be inadequate, this must be clearly noted on the quotation.

• A statement must be provided to confirm whether or not delivery is included.

• VAT must be clearly included on the quotation• The estimator (or any member of the company) must not

knowingly mislead the customer with respect to quotations or any item shown on the quotations.

THE LETTER OF APPOINTMENT OF A DESIGNER WHO WILL APPLY THE SYSTEM SOFTWARE FOR DESIGN OF A ROOF STRUCTURE WILL ENSURE:

• That all requirements pertaining to the estimator function are complied with.

• The design will include all structural elements to be supplied by the fabricator and will include not only trusses, but any beams, all necessary bracing, rafters or joists and their connections. The fabricator must ensure that the following items are complied with:

• Design must be carried out in accordance with the National Building Regulations, SANS 10160, SANS 10163, SANS 10243 and the ITC-SA recommendations for Truss Analysis and Design.

• Designs which are not of a conventional domestic nature shall be checked by the Systems Registered Professional Engineer before manufacture. Such designs may include roof or floor systems in the following categories of A (high Risk), B (Medium Risk), C (Low Risk) and D.

• The designer must check that any member sizes and grades specified by the client or his architect are adequate.

• The client or his engineer must be informed of any unusual loads or forces (vertical or horizontal) which may affect the supporting structure. Examples of loads in the category may include point loads applied to walls which may not have been designed as load bearing in addition to girder bearings which may affect lintels and foundations.

• The designs of trusses which contain elements of 38 x 50mm must be checked by the Systems Registered Professional Engineer. In addition, the use of such sizes must be clearly stated on the quotation. Where 38 x 50mm material is specified in the design, the material must be mechanically stress-graded or proof-graded and assigned working stresses in bending, tension and compression, to the satisfaction of the SABS or CSIR and the supplying mill must provide proof of its SANS permit to structurally grade such a sized timber element.

• The designer must make every reasonable effort to ensure that the design is in accordance with the most recent version of the architect’s drawings. Any significant variation or discrepancy between the latest drawings and those used to prepare the quotation must be reported to the client as soon as possible after it is noticed. The client should be made aware in writing that cost variations may arise.

THE MANUFACTURE OF MONOPLANER PREFABRICATED TIMBER ROOF TRUSSES

SANS 1900 furthermore determines the minimum standard for the manufacture of monoplaner prefabricated timber roof trusses in terms of the following:

• The roof truss material and final product• Inspection of the manufactured product before leaving the

fabricator’s yard• Methods of testing• Packing or loading and marking

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IN TERMS OF THE ABOVE, NOTABLE MENTION CAN BE MADE OF:

TIMBER: Each member of timber shall be structurally graded and the maximum difference in thickness of members in a joint area shall be 1.0mm. There shall be no wane, dead knots or resin infiltration within the connector plate area unless a larger plate is used to compensate for the ineffective area.

NAIL PLATES: Nail plates must have a firm and even contact between the faces of the timber and the nail plate. The teeth must not be flattened and ALL the teeth must be fully embedded in the members of a joint with a maximum clearance on 1mm in a member where the member is subject to the maximum tolerance in terms of thickness.

JOINT CLEARANCE: In open joints, and due to inaccuracies in cutting and assembly, the joint clearance shall not exceed 2mm, except in joint lengths exceeding 300mm, a tolerance of 5mm at one end and 0mm at the other end of the joint will be allowed.

PERMISSIBLE DEVIATIONS: Span = 10mm, height of individual trusses = 20mm

MARKINGS: Each truss shall bear the manufacturer’s name or trade mark, the last two digits of the year of manufacture and the batch or job number.

THE INSTITUTE FOR TIMBER CONSTRUCTION (ITC-SA) CATEGORIES:

The ITC-SA has determined the following categories of competence for fabricators:

Category A: Very complex roofs including scissor trusses greater than 5m span, site splicing, attics and dormers, other spans greater than 10m, piggy back trusses, cantilevers greater than 2m, complex industrial and commercial structures, laminated timber structures and, most importantly, public buildings and schools.

Category B: Complex domestic and simple industrial and commercial roofs, hips and valleys up to 10m span, non-standard loads and scissor trusses up to 5m span.

Category C: Simple roofs up to 9m span, valleys, girders and support cleats, stub ends, cantilevers up to 2m, simple hips to 9m span and minimum pitch of 15 degrees.

Category D: Gable to gable roofs with no hips and valleys up to 6.5m span.

A category B fabricator may not manufacture a category A roof, and similarly a category C fabricator may not produce a category B structure unless the design has been checked and confirmed in writing by the fabricator’s relevant software system and resident Professional Engineer.

In the case of Public Buildings (as defined by SANS 10400) all roof structure designs must be checked and confirmed by the software system even if the fabricator is classed by the ITC-SA as a category A graded fabricator.

Nail plate pulling out.

A FABRICATOR SHOULD THEREFORE:

• Be a professionally registered ITC-SA fabricator member with a current certificate of competence (COC).

• Be able to provide documentation in support of category classifications and design confirmations before the order is placed on the fabricator.

• Be able to prove that a clear and level surface inside his facility is available in order to manufacture both the lengths and heights of the largest truss in the batch or order.

• Assume responsibility for the quality of the manufactured product and product liability insurance must be proved.

• Prove that the correct storage, transport and erecting procedures are used, as timber trusses can easily bend out of plane during handling, transport and erecting and which may impact the structural integrity of the roof structure. The use of specially designed trailers will always be the preferred method of transport.

INSPECTIONS OF ERECTED ROOF STRUCTURES:

A competent person in terms of SANS 10400 part L (ROOFS) is defined as a person who:

1. is registered in terms of the Engineering Profession Act, 2000 (Act no 46 of 2000) or

2. is registered in terms of the Architectural Profession Act, 2000 (Act no 44 of 2000) and

3. is generally recognized as having the necessary experience and training to undertake rational assessments or rational designs in the field of roofs and roofing.

It is a common misconception that the issue of the so-called ‘Engineers certificate’ (or various permutations thereof) for roofs in terms of Regulation A19, Form 4, as contained in SANS 10400 part A means that the structure is designed correctly and by a person deemed to be competent. This is not necessarily the case and in the overriding majority of cases it merely confirms that the roof structure has been erected according to the design intent.

The building owner, financier and insurer could assume the risk for a structure that has not necessarily been designed and erected correctly and under supervision of a recognized competent person.

MINIMUM REQUIREMENTS:

By no means of negating any of the preceding issues, as a minimum requirement, the following should be required by the specifier and consumer in order to satisfy both the requirements of the ITC-SA, of Regulation A19 in respect of roofs and the risks assumed by financiers and insurers:

• The issue of a Truss Manufacturers Certificate by an authorized designer of the fabricator and who is proved to be competent by virtue of the issue of his certificate of competence.

• Proof of the fabricator’s valid certificate of competence in his applicable category.

• Proof of the fabricator’s defective product liability insurance.

• Confirmation by a competent person who is generally recognized as having the necessary experience and training to undertake rational assessments or rational designs in the field of roofs and roofing that the roof structure has been erected according to the design intent.

• Preferably a fabricator who operates under the SANS1900 quality scheme (SABS or SATAS) and who is permitted to apply either mark to its manufactured product.

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FINAL THOUGHTS AND GUIDANCE:

The existence of ‘roof truss brokers’ within the South African truss manufacturing landscape raises a number of concerns. The following aspects are contended to be pertinent and deserve particular regard when agreements for the manufacturing of roof trusses are facilitated by roof truss brokers:

• Consumers should comprehensively acquaint themselves with the wording of agreements entered into with brokers.

• Consumers should insist that agreements with or facilitated by brokers contain a clause detailing warranties, insurance and liabilities.

• It remains advisable that agreements for the manufacturing of trusses are entered into between the consumer and the roof truss fabricator directly, to ensure maximum protection for the consumer. However, in the event that a broker does facilitate the related agreement, it is important that the parties related to the agreement include the consumer and the fabricator. Should the consumer and the broker be the sole parties to the related agreement, the consumer may only have right of recourse against the broker and not against the fabricator, leaving the consumer at risk.

“One's philosophy is not best expressed in words; it is expressed in the choices one makes... and the choices we make are ultimately our responsibility” – Eleanor Roosevelt.

FOR MORE INFORMATION, CONTACT:

(c) 082 441 5026 | 0861 LCPROOF(e) [email protected] (w) www.lcproofing.co.za

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